Current mastalgia assessment diaries are effective in reporting severity and frequency; however, literature suggests that timing and location should also be considered when assessing mastalgia alongside severity and frequency. This study aimed to assess validity, reliability, acceptability and minimal detectable change (MDC) of an improved mastalgia diary. Twenty premenopausal females who self-reported pre-menstrual mastalgia completed the diary once a day using paper, email or mobile formats, over one menstrual cycle. Predictive validity was assessed comparing pain pre-and post-menstruation. Test-retest and internal consistency established reliability. Acceptability was assessed using evaluation questions. MDC was calculated using a previously established method using the SEM to a 95% confidence interval. Results showed pre-menstrual mastalgia was significantly higher than post-menstrual, demonstrating diary validity.Reliability tests determined high test-retest reliability (ICC > 0.90) and internal consistency (α = 0.89). The diary was acceptable for >90% of participants. MDC determined change of 1 on each question would be greater than measurement error and therefore representing ‘real’ change. This improved mastalgia diary is a more comprehensive, valid, reliable and acceptable tool for assessing mastalgia.